restyle add-wallet tabs: 'From' prefix, underline tab style

- Tab labels: 'From Phrase', 'From Key', 'From xprv'
- Visual: bottom-border underline on active tab (not filled buttons)
- Inactive tabs: muted text with hover highlight
- Container: bottom border connects tabs to content area

src/popup/views/addWallet.js

@@ -27,8 +27,11 @@ function switchMode(mode) {
     currentMode = mode;
     for (const m of MODES) {
         $("add-wallet-section-" + m).classList.toggle("hidden", m !== mode);
-        $("tab-" + m).classList.toggle("bg-fg", m === mode);
-        $("tab-" + m).classList.toggle("text-bg", m === mode);
+        const tab = $("tab-" + m);
+        tab.classList.toggle("border-fg", m === mode);
+        tab.classList.toggle("font-bold", m === mode);
+        tab.classList.toggle("border-transparent", m !== mode);
+        tab.classList.toggle("text-muted", m !== mode);
     }
     $("add-wallet-password-hint").textContent = PASSWORD_HINTS[mode];
 }